One type of electrostatographic reproducing machine is a xerographic copier or printer. In a typical xerographic copier or printer, a photoreceptor surface, for example that of a drum, is generally arranged to move in an endless path through the various processing stations of the xerographic process. As in most xerographic machines, a light image of an original document is projected or scanned onto a uniformly charged surface of a photoreceptor to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged powdered developing material called toner to form a toner image corresponding to the latent image on the photoreceptor surface. When the photoreceptor surface is reusable, the toner image is then electrostatically transferred to a recording medium, such as paper, and the surface of the photoreceptor is cleaned and prepared to be used once again for the reproduction of a copy of an original. The paper with the powdered toner thereon in imagewise configuration is separated from the photoreceptor and moved through a fuser apparatus to permanently fix or fuse the toner image to the paper.
Typically, a fuser apparatus of the type provides a combination of heat and pressure to fix the toner image on the paper. The basic architecture of a fuser apparatus is well known. Essentially, it comprises a pressure roll that rolls against a rotatable heated fuser roll to form a nip therebetween. A sheet of paper carrying an unfused or powder toner image is passed through the nip. The side of the paper having the unfused or powder toner image typically faces the fuser roll, which is often supplied with a heat source, such as a resistance heater, at the core thereof. The combination of heat from the fuser roll and pressure between the fuser roll and the pressure roll fuses the toner image to the paper, and once the fused toner cools, the image is permanently fixed to the paper.
Examples of conventional fusing systems can be found in U.S. Pat. No. 6,407,366 issued Jun. 18, 2002 and entitled “Image heating apparatus having a plurality of heat generating elements”. For the purpose of having only a small number of semiconductor switching elements, this reference discloses long heating elements that are treated similar to lamps in that they are multiple long elements parallel to the long axis, and turned on and off like lamps depending on whether the job runs on letter size or legal size sheets.
U.S. Pat. No. 6,734,397 issued May 11, 2004 and entitled “Heater having at least one cycle path resistor and image heating apparatus therein” discloses a heater, or an image heating apparatus including a heater that has a substrate, heat generating resistors formed at least in a cycle path on the substrate, and current supply electrodes provided at electrical ends of the heat generating resistors, wherein plural heat generating resistors are connected in parallel to at least one of the current supply electrodes. Thus there can be obtained a heater having excellent heat generating characteristics even in a compact dimension and an image heating apparatus utilizing such heater.
In most fusing systems in use today, such as those disclosed in the references cited above, the fusing system ordinarily suffers from lack of precise axial thermal uniformity, particularly in fusing systems being required to run at relatively higher and higher throughput speeds. Such a lack of precise axial thermal uniformity is particularly true when large jobs requiring the reproduction of many copies of a non-uniform developer mass per unit area (dma) are run through such fusing systems. This is a problem because on each sheet being fused by the system, image areas with higher densities of developed toner, or higher (dma) developer mass, tend to draw relatively more heat from the heated fuser member of the fusing system than areas with less (dma) developer mass, or less developed toner densities. The undesirable result or consequence is a fusing system with relatively hot and relatively cooler spots, which then cause subsequent inconsistent fusing and poor image quality.
Additionally, there are toner documents that may be created with regular toner in most of the document areas, and MICR (magnetic image character recognition) toner only in a few areas of the document. MICR toner ordinarily requires higher fusing temperatures than ordinary toner. Conventional fusing devices and apparatus ordinarily can only fuse at a single temperature.